The present invention relates generally to bearing assemblies and more specifically to bearing assemblies which are particularly useful in X-Ray tubes.
In an X-ray tube, electrons are produced at a cathode by heating a filament. The electrons are attracted to an anode target by a high voltage potential difference (typically about forty to one hundred fifty kilovolts). When the accelerated electrons hit the anode target, X-rays are produced. Only about one percent of the electron energy is converted into X-ray radiation. The remaining energy is converted into heat. To avoid exceeding the melting point of the focal spot on the target where electrons hit, the target is rotated. The higher the target speed, the lower the focal spot temperature.
To avoid scattering the electron beam, the cathode and anode are kept in vacuum conditions (typically about 5.times.10.sup.-7 Torr). As a result, a mechanical bearing can not be effectively supplied with oil or grease lubrication. In addition, heat generated by the interaction of the high speed electrons from the cathode with the anode target results in the operating temperature of the bearings being in the three hundred to four hundred fifty degrees centigrade range. Due to the vacuum environment and the high operating temperature, a pure, solid silver coating is usually used as a bearing lubricant. The increase of friction due to lack of liquid lubricant results in extra noise, vibration, and wear, and shortens bearing lifetime. Furthermore, the silver coated bearings have limited the speed of the anode to about ten thousand rotations per minute.
Increasing the target speed beyond ten thousand rotations per minute would provide two major advantages: (1) permitting an increase of the tube electron current (and hence the tube power) without exceeding the focal spot temperature limit; and (2) permitting an increase of the assembly temperature for the same focal spot temperature and thus increasing the number of exposures during exams or the number of patients that can be examined during a given time period.
Active magnetic bearings have been proposed for reducing noise, vibration and wear of ball bearings and for increasing X-ray tube speeds. As one example, Cornelissen et al., U.S. Pat. No. 4,322,624, describes several axial magnetic bearings situated about the anode target axial shaft. The position of the shaft is sensed with a transducer during operation. Transducers can create unreliability and can add to the complexity of associated control logic and drive systems. Another limitation with these embodiments is that designing a contact for the current path is difficult.
In some magnetic bearing assemblies, such as shown by Kirk et al., "Performance of a Magnetically Suspended Flywheel Energy Storage System," Fourth International Symposium on Magnetic Bearings, August 1994, ETH Zurich, pp. 547-552, mechanical bearings are used for touchdown purposes or as back up bearings in the event that a magnetic bearing fails. These embodiments include a touchdown gap (typically about 0.1 mm) between the backup mechanical bearings and the rotor or stator. Consequently, the rotor can be displaced with respect to the stator, and closed loop, feedback position control must be used.